Ultrasound imaging devices have become an important part of medical technology. The most commonly familiar applications for these devices are fetal imaging and cardiac imaging. The transceiver of an ultrasound imaging system is typically housed in a probe that is placed over a portion of the imaging subject's body.
An ultrasound transceiver is generally an array of piezoelectric elements. A pulse of electricity applied to ultrasound material will physically perturb it, producing a sound wave. In reverse, a sound wave striking ultrasound material will create a pulse of electricity. In an ultrasound imaging system, electrical driver circuitry perturbs the ultrasound elements with pulses of electricity. The ultrasound beam thus created reflects from the tissue of the imaging subject and returns to the transceiver, creating an "echo" signal. This signal is sampled to produce a time stream of data. The time at which any particular sample is collected is proportional to the distance (i.e. range) from the transceiver to the tissue represented by the sample.
Typically, an ultrasound array is electronically focused and steered. This means that the beam direction is determined by setting the amplification and relative phase relationship of each piezoelectric element. Because a present day ultrasound transceiver is typically a one dimensional, linear array of elements, the beam can only be steered in one angular dimension, thereby defining a single scan plane and constraining the data collection to a 2-dimensional cut as described above. If the beam could be steered in two angular dimensions, it would be possible to gather data in two angular dimensions and in range, thereby describing a volumetric portion of the imaging subject rather than a cut. This data could be displayed holographically, as a false color map or as a two dimensional image that would be rotatable in three dimensions.
A transceiver that is electronically steerable in two dimensions must include a two dimensional array of individually controllable elements. A number of problems present themselves in the construction of such an array. First, there is the problem of constructing the elements themselves. One dimensional arrays have traditionally been produced by starting with a solid piece of polarized piezoelectric material and forming individual linear elements by sawing cuts (referred to as "kerfs" in the micromachining field) into this material with a dicing saw. This process generally is too destructive for the production of a two dimensional array. Production using an excimer laser has been tried, but the problems of focusing and time-controlling this type of laser proved so great that only a limited success was achieved.
In addition, to operate correctly, the piezoelectric material of each element must be polarized. It is far more economical and more effective to begin with a solid piece of polarized dielectric material and then machine it without disturbing the polarization. The heat produced by machining with an excimer laser tends to destroy the polarization of the piezoelectric material.
A problem shared by efforts to construct such an array with virtually any sort of energy beam is that the kerfs tend to have a v-shaped cross section. Because the elements are optimally spaced .lambda./2 apart, where .lambda. is the wavelength of the transmitted ultrasound, the v-shaped kerfs deprive the user of part of the potential maximum .lambda..sup.2 /4 surface area of each element. This forces the use of a larger transmit voltage pulse for the production of the same volume of ultrasound per element and reduces the receive sensitivity of each element.
The problem of connecting each piezoelectric element of an array to driver and amplifier circuitry is also a challenge. A typical ultrasound frequency is 5 MHz, which is equivalent to a wavelength of 300 .mu.m and necessitates a two dimensional element with a transceiving surface of 150 .mu.m square. Because of this small element cross section, each element presents a high impedance to the electrodes that are placed across it. The longer the electrical leads, the more transmission line problems, such as reflection and cross-talk, will be encountered. In addition, it is a challenge to simply extend a lead to each element because in a sizable two dimensional array, the path of many leads must intersect.